The measurement of creatinine has been performed in the field of clinical care because creatinine is useful as an indicator of kidney function.
Creatinine can be measured generally by a chemical or enzymatic method. Examples of the chemical method include a Jaffe method (see, e.g., Bonsnes and Taussky, Journal of Biological Chemistry, 1945, 158, p. 581) and a Benedict-Behre method (see, e.g., Benedict and Behre, Journal of Biological Chemistry, 1936, 113, p. 515). When creatinine reacts with picric acid under alkaline conditions, it changes color to orange-red. The Jaffe method utilizes this property of creatinine, and the Benedict-Behre method uses 3,5-dinitrobenzoic acid instead of the picric acid. Examples of the enzymatic method include a creatinine deaminase method and a creatinine amidohydrolase (creatininase) method (see, e.g., Tanganelli et al., Clinical Chemistry, 1982, 28, p. 1461). In the creatinine deaminase method, an enzyme acts on creatinine to produce ammonia, and the ammonia is measured calorimetrically. In the creatinine amidohydrolase method, creatinine is converted into creatine using creatininase, and the creatine is treated with sarcosine oxidase or peroxidase and then is measured calorimetrically.